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      in Environment

      A new way to quantify climate change impacts: “Outdoor days”

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      For most people, reading about the difference between a global average temperature rise of 1.5 C versus 2 C doesn’t conjure up a clear image of how their daily lives will actually be affected. So, researchers at MIT have come up with a different way of measuring and describing what global climate change patterns, in specific regions around the world, will mean for people’s daily activities and their quality of life.

      The new measure, called “outdoor days,” describes the number of days per year that outdoor temperatures are neither too hot nor too cold for people to go about normal outdoor activities, whether work or leisure, in reasonable comfort. Describing the impact of rising temperatures in those terms reveals some significant global disparities, the researchers say.

      The findings are described in a research paper written by MIT professor of civil and environmental engineering Elfatih Eltahir and postdocs Yeon-Woo Choi and Muhammad Khalifa, and published in the Journal of Climate.

      Eltahir says he got the idea for this new system during his hourlong daily walks in the Boston area. “That’s how I interface with the temperature every day,” he says. He found that there have been more winter days recently when he could walk comfortably than in past years. Originally from Sudan, he says that when he returned there for visits, the opposite was the case: In winter, the weather tends to be relatively comfortable, but the number of these clement winter days has been declining. “There are fewer days that are really suitable for outdoor activity,” Eltahir says.

      Rather than predefine what constitutes an acceptable outdoor day, Eltahir and his co-authors created a website where users can set their own definition of the highest and lowest temperatures they consider comfortable for their outside activities, then click on a country within a world map, or a state within the U.S., and get a forecast of how the number of days meeting those criteria will change between now and the end of this century. The website is freely available for anyone to use.

      “This is actually a new feature that’s quite innovative,” he says. “We don’t tell people what an outdoor day should be; we let the user define an outdoor day. Hence, we invite them to participate in defining how future climate change will impact their quality of life, and hopefully, this will facilitate deeper understanding of how climate change will impact individuals directly.”

      After deciding that this was a way of looking at the issue of climate change that might be useful, Eltahir says, “we started looking at the data on this, and we made several discoveries that I think are pretty significant.”

      First of all, there will be winners and losers, and the losers tend to be concentrated in the global south. “In the North, in a place like Russia or Canada, you gain a significant number of outdoor days. And when you go south to places like Bangladesh or Sudan, it’s bad news. You get significantly fewer outdoor days. It is very striking.”

      To derive the data, the software developed by the team uses all of the available climate models, about 50 of them, and provides output showing all of those projections on a single graph to make clear the range of possibilities, as well as the average forecast.

      When we think of climate change, Eltahir says, we tend to look at maps that show that virtually everywhere, temperatures will rise. “But if you think in terms of outdoor days, you see that the world is not flat. The North is gaining; the South is losing.”

      While North-South disparity in exposure and vulnerability has been broadly recognized in the past, he says, this way of quantifying the effects on the hazard (change in weather patterns) helps to bring home how strong the uneven risks from climate change on quality of life will be. “When you look at places like Bangladesh, Colombia, Ivory Coast, Sudan, Indonesia — they are all losing outdoor days.”

      The same kind of disparity shows up in Europe, he says. The effects are already being felt, and are showing up in travel patterns: “There is a shift to people spending time in northern European states. They go to Sweden and places like that instead of the Mediterranean, which is showing a significant drop,” he says.

      Placing this kind of detailed and localized information at people’s fingertips, he says, “I think brings the issue of communication of climate change to a different level.” With this tool, instead of looking at global averages, “we are saying according to your own definition of what a pleasant day is, [this is] how climate change is going to impact you, your activities.”

      And, he adds, “hopefully that will help society make decisions about what to do with this global challenge.”

      The project received support from the MIT Climate Grand Challenges project “Jameel Observatory – Climate Resilience Early Warning System Network,” as well as from the Abdul Latif Jameel Water and Food Systems Lab. More

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      in Environment

      Think globally, rebuild locally

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      Building construction accounts for a huge chunk of greenhouse gas emissions: About 36 percent of carbon dioxide emissions and 40 percent of energy consumption in Europe, for instance. That’s why the European Union has developed regulations about the reuse of building materials.

      Some cities are adding more material reuse into construction already. Amsterdam, for example, is attempting to slash its raw material use by half by 2030. The Netherlands as a whole aims for a “circular economy” of completely reused materials by 2050.

      But the best way to organize the reuse of construction waste is still being determined. For one thing: Where should reusable building materials be stored before they are reused? A newly published study focusing on Amsterdam finds the optimal material reuse system for construction has many local storage “hubs” that keep materials within a few miles of where they will be needed.

      “Our findings provide a starting point for policymakers in Amsterdam to strategize land use effectively,” says Tanya Tsui, a postdoc at MIT and a co-author of the new paper. “By identifying key locations repeatedly favored across various hub scenarios, we underscore the importance of prioritizing these areas for future circular economy endeavors in Amsterdam.”

      The study adds to an emerging research area that connects climate change and urban planning.

      “The issue is where you put material in between demolition and new construction,” says Fábio Duarte, a principal researcher at MIT’s Senseable City Lab and a co-author of the new paper. “It will have huge impacts in terms of transportation. So you have to define the best sites. Should there be only one? Should we hold materials across a wide number of sites? Or is there an optimal number, even if it changes over time? This is what we examined in the paper.”

      The paper, “Spatial optimization of circular timber hubs,” is published in NPJ Nature Urban Sustainability. The authors are Tsui, who is a postdoc at the MIT Senseable Amsterdam Lab in the Amsterdam Institute for Advanced Metropolitan Solutions (AMS); Titus Venverloo, a research fellow at MIT Senseable Amsterdam Lab and AMS; Tom Benson, a researcher at the Senseable City Lab; and Duarte, who is also a lecturer in MIT’s Department of Urban Studies and Planning and the MIT Center for Real Estate.

      Numerous experts have previously studied at what scale the “circular economy” of reused materials might best operate. Some have suggested that very local circuits of materials recycling make the most sense; others have proposed that building-materials recycling will work best at a regional scale, with a radius of distribution covering 30 or more miles. Some analyses contend that global-scale reuse will be necessary to an extent.

      The current study adds to this examination of the best geographic scale for using construction materials again. Currently the storage hubs that do exist for such reused materials are chosen by individual companies, but those locations might not be optimal either economically or environmentally. 

      To conduct the study, the researchers essentially conducted a series of simulations of the Amsterdam metropolitan area, focused exclusively on timber reuse. The simulations examined how the system would work if anywhere from one to 135 timber storage hubs existed in greater Amsterdam. The modeling accounted for numerous variables, such as emissions reductions, logistical factors, and even how changing supply-and-demand scenarios would affect the viability of the reusehubs.

      Ultimately, the research found that Amsterdam’s optimal system would have 29 timber hubs, each serving a radius of about 2 miles. That setup generated 95 percent of the maximum reduction in CO2 emissions, while retaining logistical and economic benefits.

      That results also lands firmly on the side of having more localized networks for keeping construction materials in use.

      “If we have demolition happening in certain sites, then we can project where the best spots around the city are to have these circular economy hubs, as we call them,” Duarte says. “It’s not only one big hub — or one hub per construction site.”

      The study seeks to identify not only the optimal number of storage sites, but to identify where those sites might be.

      “[We hope] our research sparks discussions regarding the location and scale of circular hubs,” Tsui says. “While much attention has been given to governance aspects of the circular economy in cities, our study demonstrates the potential of utilizing location data on materials to inform decisions in urban planning.”

      The simulations also illuminated the dynamics of materials reuse. In scenarios where Amsterdam had from two to 20 timber recycling hubs, the costs involved lowered as the number of hubs increased — because having more hubs reduces transportation costs. But when the number of hubs went about 40, the system as a whole became more expensive — because each timber depot was not storing enough material to justify the land use.

      As such, the results may be of interest to climate policymakers, urban planners, and business interests getting involved in implementing the circular economy in the construction industry.

      “Ultimately,” Tsui says, “we envision our research catalyzing meaningful discussions and guiding policymakers toward more informed decisions in advancing the circular economy agenda in urban contexts.”

      The research was supported, in part, by the European Union’s Horizon 2020 research and innovation program. More

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      in Environment

      Understanding the impacts of mining on local environments and communities

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      Hydrosocial displacement refers to the idea that resolving water conflict in one area can shift the conflict to a different area. The concept was coined by Scott Odell, a visiting researcher in MIT’s Environmental Solutions Initiative (ESI). As part of ESI’s Program on Mining and the Circular Economy, Odell researches the impacts of extractive industries on local environments and communities, especially in Latin America. He discovered that hydrosocial displacements are often in regions where the mining industry is vying for use of precious water sources that are already stressed due to climate change.

      Odell is working with John Fernández, ESI director and professor in the Department of Architecture, on a project that is examining the converging impacts of climate change, mining, and agriculture in Chile. The work is funded by a seed grant from MIT’s Abdul Latif Jameel Water and Food Systems Lab (J-WAFS). Specifically, the project seeks to answer how the expansion of seawater desalination by the mining industry is affecting local populations, and how climate change and mining affect Andean glaciers and the agricultural communities dependent upon them.By working with communities in mining areas, Odell and Fernández are gaining a sense of the burden that mining minerals needed for the clean energy transition is placing on local populations, and the types of conflicts that arise when water sources become polluted or scarce. This work is of particular importance considering over 100 countries pledged a commitment to the clean energy transition at the recent United Nations climate change conference, known as COP28.

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      J-WAFS Community Spotlight on Scott Odell

      Water, humanity’s lifebloodAt the March 2023 United Nations (U.N.) Water Conference in New York, U.N. Secretary-General António Guterres warned “water is in deep trouble. We are draining humanity’s lifeblood through vampiric overconsumption and unsustainable use and evaporating it through global heating.” A quarter of the world’s population already faces “extremely high water stress,” according to the World Resources Institute. In an effort to raise awareness of major water-related issues and inspire action for innovative solutions, the U.N. created World Water Day, observed every year on March 22. This year’s theme is “Water for Peace,” underscoring the fact that even though water is a basic human right and intrinsic to every aspect of life, it is increasingly fought over as supplies dwindle due to problems including drought, overuse, or mismanagement.  

      The “Water for Peace” theme is exemplified in Fernández and Odell’s J-WAFS project, where findings are intended to inform policies to reduce social and environmental harms inflicted on mining communities and their limited water sources.“Despite broad academic engagement with mining and climate change separately, there has been a lack of analysis of the societal implications of the interactions between mining and climate change,” says Odell. “This project is helping to fill the knowledge gap. Results will be summarized in Spanish and English and distributed to interested and relevant parties in Chile, ensuring that the results can be of benefit to those most impacted by these challenges,” he adds.

      The effects of mining for the clean energy transition

      Global climate change is understood to be the most pressing environmental issue facing humanity today. Mitigating climate change requires reducing carbon emissions by transitioning away from conventional energy derived from burning fossil fuels, to more sustainable energy sources like solar and wind power. Because copper is an excellent conductor of electricity, it will be a crucial element in the clean energy transition, in which more solar panels, wind turbines, and electric vehicles will be manufactured. “We are going to see a major increase in demand for copper due to the clean energy transition,” says Odell.

      In 2021, Chile produced 26 percent of the world’s copper, more than twice as much as any other country, Odell explains. Much of Chile’s mining is concentrated in and around the Atacama Desert — the world’s driest desert. Unfortunately, mining requires large amounts of water for a variety of processes, including controlling dust at the extraction site, cooling machinery, and processing and transporting ore.

      Chile is also one of the world’s largest exporters of agricultural products. Farmland is typically situated in the valleys downstream of several mines in the high Andes region, meaning mines get first access to water. This can lead to water conflict between mining operations and agricultural communities. Compounding the problem of mining for greener energy materials to combat climate change, are the very effects of climate change. According to the Chilean government, the country has suffered 13 years of the worst drought in history. While this is detrimental to the mining industry, it is also concerning for those working in agriculture, including the Indigenous Atacameño communities that live closest to the Escondida mine, the largest copper mine in the world. “There was never a lot of water to go around, even before the mine,” Odell says. The addition of Escondida stresses an already strained water system, leaving Atacameño farmers and individuals vulnerable to severe water insecurity.

      What’s more, waste from mining, known as tailings, includes minerals and chemicals that can contaminate water in nearby communities if not properly handled and stored. Odell says the secure storage of tailings is a high priority in earthquake-prone Chile. “If an earthquake were to hit and damage a tailings dam, it could mean toxic materials flowing downstream and destroying farms and communities,” he says.

      Chile’s treasured glaciers are another piece of the mining, climate change, and agricultural puzzle. Caroline White-Nockleby, a PhD candidate in MIT’s Program in Science, Technology, and Society, is working with Odell and Fernández on the J-WAFS project and leading the research specifically on glaciers. “These may not be the picturesque bright blue glaciers that you might think of, but they are, nonetheless, an important source of water downstream,” says White-Nockleby. She goes on to explain that there are a few different ways that mines can impact glaciers.

      In some cases, mining companies have proposed to move or even destroy glaciers to get at the ore beneath. Other impacts include dust from mining that falls on glaciers. White-Nockleby says, “this makes the glaciers a darker color, so, instead of reflecting the sun’s rays away, [the glacier] may absorb the heat and melt faster.” This shows that even when not directly intervening with glaciers, mining activities can cause glacial decline, adding to the threat glaciers already face due to climate change. She also notes that “glaciers are an important water storage facility,” describing how, on an annual cycle, glaciers freeze and melt, allowing runoff that downstream agricultural communities can utilize. If glaciers suddenly melt too quickly, flooding of downstream communities can occur.

      Desalination offers a possible, but imperfect, solution

      Chile’s extensive coastline makes it uniquely positioned to utilize desalination — the removal of salts from seawater — to address water insecurity. Odell says that “over the last decade or so, there’s been billions of dollars of investments in desalination in Chile.”

      As part of his dissertation work at Clark University, Odell found broad optimism in Chile for solving water issues in the mining industry through desalination. Not only was the mining industry committed to building desalination plants, there was also political support, and support from some community members in highland communities near the mines. Yet, despite the optimism and investment, desalinated water was not replacing the use of continental water. He concluded that “desalination can’t solve water conflict if it doesn’t reduce demand for continental water supplies.”

      However, after publishing those results, Odell learned that new estimates at the national level showed that desalination operations had begun to replace the use of continental water after 2018. In two case studies that he currently focuses on — the Escondida and Los Pelambres copper mines — the mining companies have expanded their desalination objectives in order to reduce extraction from key continental sources. This seems to be due to a variety of factors. For one thing, in 2022, Chile’s water code was reformed to prioritize human water consumption and environmental protection of water during scarcity and in the allocation of future rights. It also shortened the granting of water rights from “in perpetuity” to 30 years. Under this new code, it is possible that the mining industry may have expanded its desalination efforts because it viewed continental water resources as less secure, Odell surmises.

      As part of the J-WAFS project, Odell has found that recent reactions have been mixed when it comes to the rapid increase in the use of desalination. He spent over two months doing fieldwork in Chile by conducting interviews with members of government, industry, and civil society at the Escondida, Los Pelambres, and Andina mining sites, as well as in Chile’s capital city, Santiago. He has spoken to local and national government officials, leaders of fishing unions, representatives of mining and desalination companies, and farmers. He observed that in the communities where the new desalination plants are being built, there have been concerns from community members as to whether they will get access to the desalinated water, or if it will belong solely to the mines.

      Interviews at the Escondida and Los Pelambres sites, in which desalination operations are already in place or under construction, indicate acceptance of the presence of desalination plants combined with apprehension about unknown long-term environmental impacts. At a third mining site, Andina, there have been active protests against a desalination project that would supply water to a neighboring mine, Los Bronces. In that community, there has been a blockade of the desalination operation by the fishing federation. “They were blockading that operation for three months because of concerns over what the desalination plant would do to their fishing grounds,” Odell says. And this is where the idea of hydrosocial displacement comes into the picture, he explains. Even though desalination operations are easing tensions with highland agricultural communities, new issues are arising for the communities on the coast. “We can’t just look to desalination to solve our problems if it’s going to create problems somewhere else” Odell advises.

      Within the process of hydrosocial displacement, interacting geographical, technical, economic, and political factors constrain the range of responses to address the water conflict. For example, communities that have more political and financial power tend to be better equipped to solve water conflict than less powerful communities. In addition, hydrosocial concerns usually follow the flow of water downstream, from the highlands to coastal regions. Odell says that this raises the need to look at water from a broader perspective.

      “We tend to address water concerns one by one and that can, in practice, end up being kind of like whack-a-mole,” says Odell. “When we think of the broader hydrological system, water is very much linked, and we need to look across the watershed. We can’t just be looking at the specific community affected now, but who else is affected downstream, and will be affected in the long term. If we do solve a water issue by moving it somewhere else, like moving a tailings dam somewhere else, or building a desalination plant, resources are needed in the receiving community to respond to that,” suggests Odell.

      The company building the desalination plant and the fishing federation ultimately reached an agreement and the desalination operation will be moving forward. But Odell notes, “the protest highlights concern about the impacts of the operation on local livelihoods and environments within the much larger context of industrial pollution in the area.”

      The power of communities

      The protest by the fishing federation is one example of communities coming together to have their voices heard. Recent proposals by mining companies that would affect glaciers and other water sources used by agriculture communities have led to other protests that resulted in new agreements to protect local water supplies and the withdrawal of some of the mining proposals.Odell observes that communities have also gone to the courts to raise their concerns. The Atacameño communities, for example, have drawn attention to over-extraction of water resources by the Escondida mine. “Community members are also pursuing education in these topics so that there’s not such a power imbalance between mining companies and local communities,” Odell remarks. This demonstrates the power local communities can have to protect continental water resources.The political and social landscape of Chile may also be changing in favor of local communities. Beginning with what is now referred to as the Estallido Social (social outburst) over inequality in 2019, Chile has undergone social upheaval that resulted in voters calling for a new constitution. Gabriel Boric, a progressive candidate, whose top priorities include social and environmental issues, was elected president during this period. These trends have brought major attention to issues of economic inequality, environmental harms of mining, and environmental justice, which is putting pressure on the mining industry to make a case for its operations in the country, and to justify the environmental costs of mining.

      What happens after the mine dries up?

      From his fieldwork interviews, Odell has learned that the development of mines within communities can offer benefits. Mining companies typically invest directly in communities through employment, road construction, and sometimes even by building or investing in schools, stadiums, or health clinics. Indirectly, mines can have spillover effects in the economy since miners might support local restaurants, hotels, or stores. But what happens when the mine closes? As one community member Odell interviewed stated: “When the mine is gone, what are we going to have left besides a big hole in the ground?”

      Odell suggests that a multi-pronged approach should be taken to address the future state of water and mining. First, he says we need to have broader conversations about the nature of our consumption and production at domestic and global scales. “Mining is driven indirectly by our consumption of energy and directly by our consumption of everything from our buildings to devices to cars,” Odell states. “We should be looking for ways to moderate our consumption and consume smarter through both policy and practice so that we don’t solve climate change while creating new environmental harms through mining.”One of the main ways we can do this is by advancing the circular economy by recycling metals already in the system, or even in landfills, to help build our new clean energy infrastructure. Even so, the clean energy transition will still require mining, but according to Odell, that mining can be done better. “Mining companies and government need to do a better job of consulting with communities. We need solid plans and financing for mine closures in place from the beginning of mining operations, so that when the mine dries up, there’s the money needed to secure tailings dams and protect the communities who will be there forever,” Odell concludes.Overall, it will take an engaged society — from the mining industry to government officials to individuals — to think critically about the role we each play in our quest for a more sustainable planet, and what that might mean for the most vulnerable populations among us. More

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      in Energy

      Lessons from Fukushima: Prepare for the unlikely

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      When a devastating earthquake and tsunami overwhelmed the protective systems at the Fukushima Dai’ichi nuclear power plant complex in Japan in March 2011, it triggered a sequence of events leading to one of the worst releases of radioactive materials in the world to date. Although nuclear energy is having a revival as a low-emissions energy source to mitigate climate change, the Fukushima accident is still cited as a reason for hesitancy in adopting it.

      A new study synthesizes information from multidisciplinary sources to understand how the Fukushima Dai’ichi disaster unfolded, and points to the importance of mitigation measures and last lines of defense — even against accidents considered highly unlikely. These procedures have received relatively little attention, but they are critical in determining how severe the consequences of a reactor failure will be, the researchers say.

      The researchers note that their synthesis is one of the few attempts to look at data across disciplinary boundaries, including: the physics and engineering of what took place within the plant’s systems, the plant operators’ actions throughout the emergency, actions by emergency responders, the meteorology of radionuclide releases and transport, and the environmental and health consequences documented since the event.

      The study appears in the journal iScience, in an open-access paper by postdoc Ali Ayoub and Professor Haruko Wainwright at MIT, along with others in Switzerland, Japan, and New Mexico.

      Since 2013, Wainwright has been leading the research to integrate all the radiation monitoring data in the Fukushima region into integrated maps. “I was staring at the contamination map for nearly 10 years, wondering what created the main plume extending in the northwest direction, but I could not find exact information,” Wainwright says. “Our study is unique because we started from the consequence, the contamination map, and tried to identify the key factors for the consequence. Other people study the Fukushima accident from the root cause, the tsunami.”

      One thing they found was that while all the operating reactors, units 1, 2, and 3, suffered core meltdowns as a result of the failure of emergency cooling systems, units 1 and 3 — although they did experience hydrogen explosions — did not release as much radiation to the environment because their venting systems essentially worked to relieve pressure inside the containment vessels as intended. But the same system in unit 2 failed badly.

      “People think that the hydrogen explosion or the core meltdown were the worst things, or the major driver of the radiological consequences of the accident,” Wainright says, “but our analysis found that’s not the case.” Much more significant in terms of the radiological release was the failure of the one venting mechanism.

      “There is a pressure-release mechanism that goes through water where a lot of the radionuclides get filtered out,” she explains. That system was effective in units 1 and 3, filtering out more than 90 percent of the radioactive elements before the gas was vented. However, “in unit 2, that pressure release mechanism got stuck, and the operators could not manually open it.” A hydrogen explosion in unit 1 had damaged the pressure relief mechanism of unit 2. This led to a breach of the containment structure and direct, unfiltered venting to the atmosphere, which, according to the new study, was what produced the greatest amount of contamination from the whole weeks-long event.

      Another factor was the timing of the attempt to vent the pressure buildup in the reactor. Guidelines at the time, and to this day in many reactors, specified that no venting should take place until the pressure inside the reactor containment vessel reached a specified threshold, with no regard to the wind directions at the time. In the case of Fukushima, an earlier venting could have dramatically reduced the impact: Much of the release happened when winds were blowing directly inland, but earlier the wind had been blowing offshore.

      “That pressure-release mechanism has not been a major focus of the engineering community,” she says. While there is appropriate attention to measures that prevent a core meltdown in the first place, “this sort of last line of defense has not been the main focus and should get more attention.”

      Wainwright says the study also underlines several successes in the management of the Fukushima accident. Many of the safety systems did work as they were designed. For example, even though the oldest reactor, unit 1, suffered the greatest internal damage, it released little radioactive material. Most people were able to evacuate from the 20-kilometer (12-mile) zone before the largest release happened. The mitigation measures were “somewhat successful,” Wainwright says. But there was tremendous confusion and anger during and after the accident because there were no preparations in place for such an event.

      Much work has focused on ways to prevent the kind of accidents that happened at Fukushima — for example, in the U.S. reactor operators can deploy portable backup power supplies to maintain proper reactor cooling at any reactor site. But the ongoing situation at the Zaporizhzhia nuclear complex in Ukraine, where nuclear safety is challenged by acts of war, demonstrates that despite engineers’ and operators’ best efforts to prevent it, “the totally unexpected could still happen,” Wainwright says.

      “The big-picture message is that we should have equal attention to both prevention and mitigation of accidents,” she says. “This is the essence of resilience, and it applies beyond nuclear power plants to all essential infrastructure of a functioning society, for example, the electric grid, the food and water supply, the transportation sector, etc.”

      One thing the researchers recommend is that in designing evacuation protocols, planners should make more effort to learn from much more frequent disasters such as wildfires and hurricanes. “We think getting more interdisciplinary, transdisciplinary knowledge from other kinds of disasters would be essential,” she says. Most of the emergency response strategies presently in place, she says, were designed in the 1980s and ’90s, and need to be modernized. “Consequences can be mitigated. A nuclear accident does not have to be a catastrophe, as is often portrayed in popular culture,” Wainright says.

      The research team included Giovanni Sansavini at ETH Zurich in Switzerland; Randall Gauntt at Sandia National Laboratories in New Mexico; and Kimiaki Saito at the Japan Atomic Energy Agency. More

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      in Energy

      Future nuclear power reactors could rely on molten salts — but what about corrosion?

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      Most discussions of how to avert climate change focus on solar and wind generation as key to the transition to a future carbon-free power system. But Michael Short, the Class of ’42 Associate Professor of Nuclear Science and Engineering at MIT and associate director of the MIT Plasma Science and Fusion Center (PSFC), is impatient with such talk. “We can say we should have only wind and solar someday. But we don’t have the luxury of ‘someday’ anymore, so we can’t ignore other helpful ways to combat climate change,” he says. “To me, it’s an ‘all-hands-on-deck’ thing. Solar and wind are clearly a big part of the solution. But I think that nuclear power also has a critical role to play.”

      For decades, researchers have been working on designs for both fission and fusion nuclear reactors using molten salts as fuels or coolants. While those designs promise significant safety and performance advantages, there’s a catch: Molten salt and the impurities within it often corrode metals, ultimately causing them to crack, weaken, and fail. Inside a reactor, key metal components will be exposed not only to molten salt but also simultaneously to radiation, which generally has a detrimental effect on materials, making them more brittle and prone to failure. Will irradiation make metal components inside a molten salt-cooled nuclear reactor corrode even more quickly?

      Short and Weiyue Zhou PhD ’21, a postdoc in the PSFC, have been investigating that question for eight years. Their recent experimental findings show that certain alloys will corrode more slowly when they’re irradiated — and identifying them among all the available commercial alloys can be straightforward.

      The first challenge — building a test facility

      When Short and Zhou began investigating the effect of radiation on corrosion, practically no reliable facilities existed to look at the two effects at once. The standard approach was to examine such mechanisms in sequence: first corrode, then irradiate, then examine the impact on the material. That approach greatly simplifies the task for the researchers, but with a major trade-off. “In a reactor, everything is going to be happening at the same time,” says Short. “If you separate the two processes, you’re not simulating a reactor; you’re doing some other experiment that’s not as relevant.”

      So, Short and Zhou took on the challenge of designing and building an experimental setup that could do both at once. Short credits a team at the University of Michigan for paving the way by designing a device that could accomplish that feat in water, rather than molten salts. Even so, Zhou notes, it took them three years to come up with a device that would work with molten salts. Both researchers recall failure after failure, but the persistent Zhou ultimately tried a totally new design, and it worked. Short adds that it also took them three years to precisely replicate the salt mixture used by industry — another factor critical to getting a meaningful result. The hardest part was achieving and ensuring that the purity was correct by removing critical impurities such as moisture, oxygen, and certain other metals.

      As they were developing and testing their setup, Short and Zhou obtained initial results showing that proton irradiation did not always accelerate corrosion but sometimes actually decelerated it. They and others had hypothesized that possibility, but even so, they were surprised. “We thought we must be doing something wrong,” recalls Short. “Maybe we mixed up the samples or something.” But they subsequently made similar observations for a variety of conditions, increasing their confidence that their initial observations were not outliers.

      The successful setup

      Central to their approach is the use of accelerated protons to mimic the impact of the neutrons inside a nuclear reactor. Generating neutrons would be both impractical and prohibitively expensive, and the neutrons would make everything highly radioactive, posing health risks and requiring very long times for an irradiated sample to cool down enough to be examined. Using protons would enable Short and Zhou to examine radiation-altered corrosion both rapidly and safely.

      Key to their experimental setup is a test chamber that they attach to a proton accelerator. To prepare the test chamber for an experiment, they place inside it a thin disc of the metal alloy being tested on top of a a pellet of salt. During the test, the entire foil disc is exposed to a bath of molten salt. At the same time, a beam of protons bombards the sample from the side opposite the salt pellet, but the proton beam is restricted to a circle in the middle of the foil sample. “No one can argue with our results then,” says Short. “In a single experiment, the whole sample is subjected to corrosion, and only a circle in the center of the sample is simultaneously irradiated by protons. We can see the curvature of the proton beam outline in our results, so we know which region is which.”

      The results with that arrangement were unchanged from the initial results. They confirmed the researchers’ preliminary findings, supporting their controversial hypothesis that rather than accelerating corrosion, radiation would actually decelerate corrosion in some materials under some conditions. Fortunately, they just happen to be the same conditions that will be experienced by metals in molten salt-cooled reactors.

      Why is that outcome controversial? A closeup look at the corrosion process will explain. When salt corrodes metal, the salt finds atomic-level openings in the solid, seeps in, and dissolves salt-soluble atoms, pulling them out and leaving a gap in the material — a spot where the material is now weak. “Radiation adds energy to atoms, causing them to be ballistically knocked out of their positions and move very fast,” explains Short. So, it makes sense that irradiating a material would cause atoms to move into the salt more quickly, increasing the rate of corrosion. Yet in some of their tests, the researchers found the opposite to be true.

      Experiments with “model” alloys

      The researchers’ first experiments in their novel setup involved “model” alloys consisting of nickel and chromium, a simple combination that would give them a first look at the corrosion process in action. In addition, they added europium fluoride to the salt, a compound known to speed up corrosion. In our everyday world, we often think of corrosion as taking years or decades, but in the more extreme conditions of a molten salt reactor it can noticeably occur in just hours. The researchers used the europium fluoride to speed up corrosion even more without changing the corrosion process. This allowed for more rapid determination of which materials, under which conditions, experienced more or less corrosion with simultaneous proton irradiation.

      The use of protons to emulate neutron damage to materials meant that the experimental setup had to be carefully designed and the operating conditions carefully selected and controlled. Protons are hydrogen atoms with an electrical charge, and under some conditions the hydrogen could chemically react with atoms in the sample foil, altering the corrosion response, or with ions in the salt, making the salt more corrosive. Therefore, the proton beam had to penetrate the foil sample but then stop in the salt as soon as possible. Under these conditions, the researchers found they could deliver a relatively uniform dose of radiation inside the foil layer while also minimizing chemical reactions in both the foil and the salt.

      Tests showed that a proton beam accelerated to 3 million electron-volts combined with a foil sample between 25 and 30 microns thick would work well for their nickel-chromium alloys. The temperature and duration of the exposure could be adjusted based on the corrosion susceptibility of the specific materials being tested.

      Optical images of samples examined after tests with the model alloys showed a clear boundary between the area that was exposed only to the molten salt and the area that was also exposed to the proton beam. Electron microscope images focusing on that boundary showed that the area that had been exposed only to the molten salt included dark patches where the molten salt had penetrated all the way through the foil, while the area that had also been exposed to the proton beam showed almost no such dark patches.

      To confirm that the dark patches were due to corrosion, the researchers cut through the foil sample to create cross sections. In them, they could see tunnels that the salt had dug into the sample. “For regions not under radiation, we see that the salt tunnels link the one side of the sample to the other side,” says Zhou. “For regions under radiation, we see that the salt tunnels stop more or less halfway and rarely reach the other side. So we verified that they didn’t penetrate the whole way.”

      The results “exceeded our wildest expectations,” says Short. “In every test we ran, the application of radiation slowed corrosion by a factor of two to three times.”

      More experiments, more insights

      In subsequent tests, the researchers more closely replicated commercially available molten salt by omitting the additive (europium fluoride) that they had used to speed up corrosion, and they tweaked the temperature for even more realistic conditions. “In carefully monitored tests, we found that by raising the temperature by 100 degrees Celsius, we could get corrosion to happen about 1,000 times faster than it would in a reactor,” says Short.

      Images from experiments with the nickel-chromium alloy plus the molten salt without the corrosive additive yielded further insights. Electron microscope images of the side of the foil sample facing the molten salt showed that in sections only exposed to the molten salt, the corrosion is clearly focused on the weakest part of the structure — the boundaries between the grains in the metal. In sections that were exposed to both the molten salt and the proton beam, the corrosion isn’t limited to the grain boundaries but is more spread out over the surface. Experimental results showed that these cracks are shallower and less likely to cause a key component to break.

      Short explains the observations. Metals are made up of individual grains inside which atoms are lined up in an orderly fashion. Where the grains come together there are areas — called grain boundaries — where the atoms don’t line up as well. In the corrosion-only images, dark lines track the grain boundaries. Molten salt has seeped into the grain boundaries and pulled out salt-soluble atoms. In the corrosion-plus-irradiation images, the damage is more general. It’s not only the grain boundaries that get attacked but also regions within the grains.

      So, when the material is irradiated, the molten salt also removes material from within the grains. Over time, more material comes out of the grains themselves than from the spaces between them. The removal isn’t focused on the grain boundaries; it’s spread out over the whole surface. As a result, any cracks that form are shallower and more spread out, and the material is less likely to fail.

      Testing commercial alloys

      The experiments described thus far involved model alloys — simple combinations of elements that are good for studying science but would never be used in a reactor. In the next series of experiments, the researchers focused on three commercially available alloys that are composed of nickel, chromium, iron, molybdenum, and other elements in various combinations.

      Results from the experiments with the commercial alloys showed a consistent pattern — one that confirmed an idea that the researchers had going in: the higher the concentration of salt-soluble elements in the alloy, the worse the radiation-induced corrosion damage. Radiation will increase the rate at which salt-soluble atoms such as chromium leave the grain boundaries, hastening the corrosion process. However, if there are more not-soluble elements such as nickel present, those atoms will go into the salt more slowly. Over time, they’ll accumulate at the grain boundary and form a protective coating that blocks the grain boundary — a “self-healing mechanism that decelerates the rate of corrosion,” say the researchers.

      Thus, if an alloy consists mostly of atoms that don’t dissolve in molten salt, irradiation will cause them to form a protective coating that slows the corrosion process. But if an alloy consists mostly of atoms that dissolve in molten salt, irradiation will make them dissolve faster, speeding up corrosion. As Short summarizes, “In terms of corrosion, irradiation makes a good alloy better and a bad alloy worse.”

      Real-world relevance plus practical guidelines

      Short and Zhou find their results encouraging. In a nuclear reactor made of “good” alloys, the slowdown in corrosion will probably be even more pronounced than what they observed in their proton-based experiments because the neutrons that inflict the damage won’t chemically react with the salt to make it more corrosive. As a result, reactor designers could push the envelope more in their operating conditions, allowing them to get more power out of the same nuclear plant without compromising on safety.

      However, the researchers stress that there’s much work to be done. Many more projects are needed to explore and understand the exact corrosion mechanism in specific alloys under different irradiation conditions. In addition, their findings need to be replicated by groups at other institutions using their own facilities. “What needs to happen now is for other labs to build their own facilities and start verifying whether they get the same results as we did,” says Short. To that end, Short and Zhou have made the details of their experimental setup and all of their data freely available online. “We’ve also been actively communicating with researchers at other institutions who have contacted us,” adds Zhou. “When they’re planning to visit, we offer to show them demonstration experiments while they’re here.”

      But already their findings provide practical guidance for other researchers and equipment designers. For example, the standard way to quantify corrosion damage is by “mass loss,” a measure of how much weight the material has lost. But Short and Zhou consider mass loss a flawed measure of corrosion in molten salts. “If you’re a nuclear plant operator, you usually care whether your structural components are going to break,” says Short. “Our experiments show that radiation can change how deep the cracks are, when all other things are held constant. The deeper the cracks, the more likely a structural component is to break, leading to a reactor failure.”

      In addition, the researchers offer a simple rule for identifying good metal alloys for structural components in molten salt reactors. Manufacturers provide extensive lists of available alloys with different compositions, microstructures, and additives. Faced with a list of options for critical structures, the designer of a new nuclear fission or fusion reactor can simply examine the composition of each alloy being offered. The one with the highest content of corrosion-resistant elements such as nickel will be the best choice. Inside a nuclear reactor, that alloy should respond to a bombardment of radiation not by corroding more rapidly but by forming a protective layer that helps block the corrosion process. “That may seem like a trivial result, but the exact threshold where radiation decelerates corrosion depends on the salt chemistry, the density of neutrons in the reactor, their energies, and a few other factors,” says Short. “Therefore, the complete guidelines are a bit more complicated. But they’re presented in a straightforward way that users can understand and utilize to make a good choice for the molten salt–based reactor they’re designing.”

      This research was funded, in part, by Eni S.p.A. through the MIT Plasma Science and Fusion Center’s Laboratory for Innovative Fusion Technologies. Earlier work was funded, in part, by the Transatomic Power Corporation and by the U.S. Department of Energy Nuclear Energy University Program. Equipment development and testing was supported by the Transatomic Power Corporation.

      This article appears in the Winter 2024 issue of Energy Futures, the magazine of the MIT Energy Initiative. More

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      in Environment

      Study finds lands used for grazing can worsen or help climate change

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      When it comes to global climate change, livestock grazing can be either a blessing or a curse, according to a new study, which offers clues on how to tell the difference.

      If managed properly, the study shows, grazing can actually increase the amount of carbon from the air that gets stored in the ground and sequestered for the long run. But if there is too much grazing, soil erosion can result, and the net effect is to cause more carbon losses, so that the land becomes a net carbon source, instead of a carbon sink. And the study found that the latter is far more common around the world today.

      The new work, published today in the journal Nature Climate Change, provides ways to determine the tipping point between the two, for grazing lands in a given climate zone and soil type. It also provides an estimate of the total amount of carbon that has been lost over past decades due to livestock grazing, and how much could be removed from the atmosphere if grazing optimization management implemented. The study was carried out by Cesar Terrer, an assistant professor of civil and environmental engineering at MIT; Shuai Ren, a PhD student at the Chinese Academy of Sciences whose thesis is co-supervised by Terrer; and four others.

      “This has been a matter of debate in the scientific literature for a long time,” Terrer says. “In general experiments, grazing decreases soil carbon stocks, but surprisingly, sometimes grazing increases soil carbon stocks, which is why it’s been puzzling.”

      What happens, he explains, is that “grazing could stimulate vegetation growth through easing resource constraints such as light and nutrients, thereby increasing root carbon inputs to soils, where carbon can stay there for centuries or millennia.”

      But that only works up to a certain point, the team found after a careful analysis of 1,473 soil carbon observations from different grazing studies from many locations around the world. “When you cross a threshold in grazing intensity, or the amount of animals grazing there, that is when you start to see sort of a tipping point — a strong decrease in the amount of carbon in the soil,” Terrer explains.

      That loss is thought to be primarily from increased soil erosion on the denuded land. And with that erosion, Terrer says, “basically you lose a lot of the carbon that you have been locking in for centuries.”

      The various studies the team compiled, although they differed somewhat, essentially used similar methodology, which is to fence off a portion of land so that livestock can’t access it, and then after some time take soil samples from within the enclosure area, and from comparable nearby areas that have been grazed, and compare the content of carbon compounds.

      “Along with the data on soil carbon for the control and grazed plots,” he says, “we also collected a bunch of other information, such as the mean annual temperature of the site, mean annual precipitation, plant biomass, and properties of the soil, like pH and nitrogen content. And then, of course, we estimate the grazing intensity — aboveground biomass consumed, because that turns out to be the key parameter.”  

      With artificial intelligence models, the authors quantified the importance of each of these parameters, those drivers of intensity — temperature, precipitation, soil properties — in modulating the sign (positive or negative) and magnitude of the impact of grazing on soil carbon stocks. “Interestingly, we found soil carbon stocks increase and then decrease with grazing intensity, rather than the expected linear response,” says Ren.

      Having developed the model through AI methods and validated it, including by comparing its predictions with those based on underlying physical principles, they can then apply the model to estimating both past and future effects. “In this case,” Terrer says, “we use the model to quantify the historical loses in soil carbon stocks from grazing. And we found that 46 petagrams [billion metric tons] of soil carbon, down to a depth of one meter, have been lost in the last few decades due to grazing.”

      By way of comparison, the total amount of greenhouse gas emissions per year from all fossil fuels is about 10 petagrams, so the loss from grazing equals more than four years’ worth of all the world’s fossil emissions combined.

      What they found was “an overall decline in soil carbon stocks, but with a lot of variability.” Terrer says. The analysis showed that the interplay between grazing intensity and environmental conditions such as temperature could explain the variability, with higher grazing intensity and hotter climates resulting in greater carbon loss. “This means that policy-makers should take into account local abiotic and biotic factors to manage rangelands efficiently,” Ren notes. “By ignoring such complex interactions, we found that using IPCC [Intergovernmental Panel on Climate Change] guidelines would underestimate grazing-induced soil carbon loss by a factor of three globally.”

      Using an approach that incorporates local environmental conditions, the team produced global, high-resolution maps of optimal grazing intensity and the threshold of intensity at which carbon starts to decrease very rapidly. These maps are expected to serve as important benchmarks for evaluating existing grazing practices and provide guidance to local farmers on how to effectively manage their grazing lands.

      Then, using that map, the team estimated how much carbon could be captured if all grazing lands were limited to their optimum grazing intensity. Currently, the authors found, about 20 percent of all pasturelands have crossed the thresholds, leading to severe carbon losses. However, they found that under the optimal levels, global grazing lands would sequester 63 petagrams of carbon. “It is amazing,” Ren says. “This value is roughly equivalent to a 30-year carbon accumulation from global natural forest regrowth.”

      That would be no simple task, of course. To achieve optimal levels, the team found that approximately 75 percent of all grazing areas need to reduce grazing intensity. Overall, if the world seriously reduces the amount of grazing, “you have to reduce the amount of meat that’s available for people,” Terrer says.

      “Another option is to move cattle around,” he says, “from areas that are more severely affected by grazing intensity, to areas that are less affected. Those rotations have been suggested as an opportunity to avoid the more drastic declines in carbon stocks without necessarily reducing the availability of meat.”

      This study didn’t delve into these social and economic implications, Terrer says. “Our role is to just point out what would be the opportunity here. It shows that shifts in diets can be a powerful way to mitigate climate change.”

      “This is a rigorous and careful analysis that provides our best look to date at soil carbon changes due to livestock grazing practiced worldwide,” say Ben Bond-Lamberty, a terrestrial ecosystem research scientist at Pacific Northwest National Laboratory, who was not associated with this work. “The authors’ analysis gives us a unique estimate of soil carbon losses due to grazing and, intriguingly, where and how the process might be reversed.”

      He adds: “One intriguing aspect to this work is the discrepancies between its results and the guidelines currently used by the IPCC — guidelines that affect countries’ commitments, carbon-market pricing, and policies.” However, he says, “As the authors note, the amount of carbon historically grazed soils might be able to take up is small relative to ongoing human emissions. But every little bit helps!”

      “Improved management of working lands can be a powerful tool to combat climate change,” says Jonathan Sanderman, carbon program director of the Woodwell Climate Research Center in Falmouth, Massachusetts, who was not associated with this work. He adds, “This work demonstrates that while, historically, grazing has been a large contributor to climate change, there is significant potential to decrease the climate impact of livestock by optimizing grazing intensity to rebuild lost soil carbon.”

      Terrer states that for now, “we have started a new study, to evaluate the consequences of shifts in diets for carbon stocks. I think that’s the million-dollar question: How much carbon could you sequester, compared to business as usual, if diets shift to more vegan or vegetarian?” The answers will not be simple, because a shift to more vegetable-based diets would require more cropland, which can also have different environmental impacts. Pastures take more land than crops, but produce different kinds of emissions. “What’s the overall impact for climate change? That is the question we’re interested in,” he says.

      The research team included Juan Li, Yingfao Cao, Sheshan Yang, and Dan Liu, all with the  Chinese Academy of Sciences. The work was supported by the Second Tibetan Plateau Scientific Expedition and Research Program, and the Science and Technology Major Project of Tibetan Autonomous Region of China. More

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      in Environment

      Reducing pesticide use while increasing effectiveness

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      Farming can be a low-margin, high-risk business, subject to weather and climate patterns, insect population cycles, and other unpredictable factors. Farmers need to be savvy managers of the many resources they deal, and chemical fertilizers and pesticides are among their major recurring expenses.

      Despite the importance of these chemicals, a lack of technology that monitors and optimizes sprays has forced farmers to rely on personal experience and rules of thumb to decide how to apply these chemicals. As a result, these chemicals tend to be over-sprayed, leading to their runoff into waterways and buildup up in the soil.

      That could change, thanks to a new approach of feedback-optimized spraying, invented by AgZen, an MIT spinout founded in 2020 by Professor Kripa Varanasi and Vishnu Jayaprakash SM ’19, PhD ’22.

      Play video

      AgZen has developed a system for farming that can monitor exactly how much of the sprayed chemicals adheres to plants, in real time, as the sprayer drives through a field. Built-in software running on a tablet shows the operator exactly how much of each leaf has been covered by the spray.

      Over the past decade, AgZen’s founders have developed products and technologies to control the interactions of droplets and sprays with plant surfaces. The Boston-based venture-backed company launched a new commercial product in 2024 and is currently piloting another related product. Field tests of both have shown the products can help farmers spray more efficiently and effectively, using fewer chemicals overall.

      “Worldwide, farms spend approximately $60 billion a year on pesticides. Our objective is to reduce the number of pesticides sprayed and lighten the financial burden on farms without sacrificing effective pest management,” Varanasi says.

      Getting droplets to stick

      While the world pesticide market is growing rapidly, a lot of the pesticides sprayed don’t reach their target. A significant portion bounces off the plant surfaces, lands on the ground, and becomes part of the runoff that flows to streams and rivers, often causing serious pollution. Some of these pesticides can be carried away by wind over very long distances.

      “Drift, runoff, and poor application efficiency are well-known, longstanding problems in agriculture, but we can fix this by controlling and monitoring how sprayed droplets interact with leaves,” Varanasi says.

      With support from MIT Tata Center and the Abdul Latif Jameel Water and Food Systems Lab, Varanasi and his team analyzed how droplets strike plant surfaces, and explored ways to increase application efficiency. This research led them to develop a novel system of nozzles that cloak droplets with compounds that enhance the retention of droplets on the leaves, a product they call EnhanceCoverage.

      Field studies across regions — from Massachusetts to California to Italy and France —showed that this droplet-optimization system could allow farmers to cut the amount of chemicals needed by more than half because more of the sprayed substances would stick to the leaves.

      Measuring coverage

      However, in trying to bring this technology to market, the researchers faced a sticky problem: Nobody knew how well pesticide sprays were adhering to the plants in the first place, so how could AgZen say that the coverage was better with its new EnhanceCoverage system?

      “I had grown up spraying with a backpack on a small farm in India, so I knew this was an issue,” Jayaprakash says. “When we spoke to growers, they told me how complicated spraying is when you’re on a large machine. Whenever you spray, there are so many things that can influence how effective your spray is. How fast do you drive the sprayer? What flow rate are you using for the chemicals? What chemical are you using? What’s the age of the plants, what’s the nozzle you’re using, what is the weather at the time? All these things influence agrochemical efficiency.”

      Agricultural spraying essentially comes down to dissolving a chemical in water and then spraying droplets onto the plants. “But the interaction between a droplet and the leaf is complex,” Varanasi says. “We were coming in with ways to optimize that, but what the growers told us is, hey, we’ve never even really looked at that in the first place.”

      Although farmers have been spraying agricultural chemicals on a large scale for about 80 years, they’ve “been forced to rely on general rules of thumb and pick all these interlinked parameters, based on what’s worked for them in the past. You pick a set of these parameters, you go spray, and you’re basically praying for outcomes in terms of how effective your pest control is,” Varanasi says.

      Before AgZen could sell farmers on the new system to improve droplet coverage, the company had to invent a way to measure precisely how much spray was adhering to plants in real-time.

      Comparing before and after

      The system they came up with, which they tested extensively on farms across the country last year, involves a unit that can be bolted onto the spraying arm of virtually any sprayer. It carries two sensor stacks, one just ahead of the sprayer nozzles and one behind. Then, built-in software running on a tablet shows the operator exactly how much of each leaf has been covered by the spray. It also computes how much those droplets will spread out or evaporate, leading to a precise estimate of the final coverage.

      “There’s a lot of physics that governs how droplets spread and evaporate, and this has been incorporated into software that a farmer can use,” Varanasi says. “We bring a lot of our expertise into understanding droplets on leaves. All these factors, like how temperature and humidity influence coverage, have always been nebulous in the spraying world. But now you have something that can be exact in determining how well your sprays are doing.”

      “We’re not only measuring coverage, but then we recommend how to act,” says Jayaprakash, who is AgZen’s CEO. “With the information we collect in real-time and by using AI, RealCoverage tells operators how to optimize everything on their sprayer, from which nozzle to use, to how fast to drive, to how many gallons of spray is best for a particular chemical mix on a particular acre of a crop.”

      The tool was developed to prove how much AgZen’s EnhanceCoverage nozzle system (which will be launched in 2025) improves coverage. But it turns out that monitoring and optimizing droplet coverage on leaves in real-time with this system can itself yield major improvements.

      “We worked with large commercial farms last year in specialty and row crops,” Jayaprakash says. “When we saved our pilot customers up to 50 percent of their chemical cost at a large scale, they were very surprised.” He says the tool has reduced chemical costs and volume in fallow field burndowns, weed control in soybeans, defoliation in cotton, and fungicide and insecticide sprays in vegetables and fruits. Along with data from commercial farms, field trials conducted by three leading agricultural universities have also validated these results.

      “Across the board, we were able to save between 30 and 50 percent on chemical costs and increase crop yields by enabling better pest control,” Jayaprakash says. “By focusing on the droplet-leaf interface, our product can help any foliage spray throughout the year, whereas most technological advancements in this space recently have been focused on reducing herbicide use alone.” The company now intends to lease the system across thousands of acres this year.

      And these efficiency gains can lead to significant returns at scale, he emphasizes: In the U.S., farmers currently spend $16 billion a year on chemicals, to protect about $200 billion of crop yields.

      The company launched its first product, the coverage optimization system called RealCoverage, this year, reaching a wide variety of farms with different crops and in different climates. “We’re going from proof-of-concept with pilots in large farms to a truly massive scale on a commercial basis with our lease-to-own program,” Jayaprakash says.

      “We’ve also been tapped by the USDA to help them evaluate practices to minimize pesticides in watersheds,” Varanasi says, noting that RealCoverage can also be useful for regulators, chemical companies, and agricultural equipment manufacturers.

      Once AgZen has proven the effectiveness of using coverage as a decision metric, and after the RealCoverage optimization system is widely in practice, the company will next roll out its second product, EnhanceCoverage, designed to maximize droplet adhesion. Because that system will require replacing all the nozzles on a sprayer, the researchers are doing pilots this year but will wait for a full rollout in 2025, after farmers have gained experience and confidence with their initial product.

      “There is so much wastage,” Varanasi says. “Yet farmers must spray to protect crops, and there is a lot of environmental impact from this. So, after all this work over the years, learning about how droplets stick to surfaces and so on, now the culmination of it in all these products for me is amazing, to see all this come alive, to see that we’ll finally be able to solve the problem we set out to solve and help farmers.” More

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      in Energy

      Cutting carbon emissions on the US power grid

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      To help curb climate change, the United States is working to reduce carbon emissions from all sectors of the energy economy. Much of the current effort involves electrification — switching to electric cars for transportation, electric heat pumps for home heating, and so on. But in the United States, the electric power sector already generates about a quarter of all carbon emissions. “Unless we decarbonize our electric power grids, we’ll just be shifting carbon emissions from one source to another,” says Amanda Farnsworth, a PhD candidate in chemical engineering and research assistant at the MIT Energy Initiative (MITEI).

      But decarbonizing the nation’s electric power grids will be challenging. The availability of renewable energy resources such as solar and wind varies in different regions of the country. Likewise, patterns of energy demand differ from region to region. As a result, the least-cost pathway to a decarbonized grid will differ from one region to another.

      Over the past two years, Farnsworth and Emre Gençer, a principal research scientist at MITEI, developed a power system model that would allow them to investigate the importance of regional differences — and would enable experts and laypeople alike to explore their own regions and make informed decisions about the best way to decarbonize. “With this modeling capability you can really understand regional resources and patterns of demand, and use them to do a ‘bespoke’ analysis of the least-cost approach to decarbonizing the grid in your particular region,” says Gençer.

      To demonstrate the model’s capabilities, Gençer and Farnsworth performed a series of case studies. Their analyses confirmed that strategies must be designed for specific regions and that all the costs and carbon emissions associated with manufacturing and installing solar and wind generators must be included for accurate accounting. But the analyses also yielded some unexpected insights, including a correlation between a region’s wind energy and the ease of decarbonizing, and the important role of nuclear power in decarbonizing the California grid.

      A novel model

      For many decades, researchers have been developing “capacity expansion models” to help electric utility planners tackle the problem of designing power grids that are efficient, reliable, and low-cost. More recently, many of those models also factor in the goal of reducing or eliminating carbon emissions. While those models can provide interesting insights relating to decarbonization, Gençer and Farnsworth believe they leave some gaps that need to be addressed.

      For example, most focus on conditions and needs in a single U.S. region without highlighting the unique peculiarities of their chosen area of focus. Hardly any consider the carbon emitted in fabricating and installing such “zero-carbon” technologies as wind turbines and solar panels. And finally, most of the models are challenging to use. Even experts in the field must search out and assemble various complex datasets in order to perform a study of interest.

      Gençer and Farnsworth’s capacity expansion model — called Ideal Grid, or IG — addresses those and other shortcomings. IG is built within the framework of MITEI’s Sustainable Energy System Analysis Modeling Environment (SESAME), an energy system modeling platform that Gençer and his colleagues at MITEI have been developing since 2017. SESAME models the levels of greenhouse gas emissions from multiple, interacting energy sectors in future scenarios.

      Importantly, SESAME includes both techno-economic analyses and life-cycle assessments of various electricity generation and storage technologies. It thus considers costs and emissions incurred at each stage of the life cycle (manufacture, installation, operation, and retirement) for all generators. Most capacity expansion models only account for emissions from operation of fossil fuel-powered generators. As Farnsworth notes, “While this is a good approximation for our current grid, emissions from the full life cycle of all generating technologies become non-negligible as we transition to a highly renewable grid.”

      Through its connection with SESAME, the IG model has access to data on costs and emissions associated with many technologies critical to power grid operation. To explore regional differences in the cost-optimized decarbonization strategies, the IG model also includes conditions within each region, notably details on demand profiles and resource availability.

      In one recent study, Gençer and Farnsworth selected nine of the standard North American Electric Reliability Corporation (NERC) regions. For each region, they incorporated hourly electricity demand into the IG model. Farnsworth also gathered meteorological data for the nine U.S. regions for seven years — 2007 to 2013 — and calculated hourly power output profiles for the renewable energy sources, including solar and wind, taking into account the geography-limited maximum capacity of each technology.

      The availability of wind and solar resources differs widely from region to region. To permit a quick comparison, the researchers use a measure called “annual capacity factor,” which is the ratio between the electricity produced by a generating unit in a year and the electricity that could have been produced if that unit operated continuously at full power for that year. Values for the capacity factors in the nine U.S. regions vary between 20 percent and 30 percent for solar power and for between 25 percent and 45 percent for wind.

      Calculating optimized grids for different regions

      For their first case study, Gençer and Farnsworth used the IG model to calculate cost-optimized regional grids to meet defined caps on carbon dioxide (CO2) emissions. The analyses were based on cost and emissions data for 10 technologies: nuclear, wind, solar, three types of natural gas, three types of coal, and energy storage using lithium-ion batteries. Hydroelectric was not considered in this study because there was no comprehensive study outlining potential expansion sites with their respective costs and expected power output levels.

      To make region-to-region comparisons easy, the researchers used several simplifying assumptions. Their focus was on electricity generation, so the model calculations assume the same transmission and distribution costs and efficiencies for all regions. Also, the calculations did not consider the generator fleet currently in place. The goal was to investigate what happens if each region were to start from scratch and generate an “ideal” grid.

      To begin, Gençer and Farnsworth calculated the most economic combination of technologies for each region if it limits its total carbon emissions to 100, 50, and 25 grams of CO2 per kilowatt-hour (kWh) generated. For context, the current U.S. average emissions intensity is 386 grams of CO2 emissions per kWh.

      Given the wide variation in regional demand, the researchers needed to use a new metric to normalize their results and permit a one-to-one comparison between regions. Accordingly, the model calculates the required generating capacity divided by the average demand for each region. The required capacity accounts for both the variation in demand and the inability of generating systems — particularly solar and wind — to operate at full capacity all of the time.

      The analysis was based on regional demand data for 2021 — the most recent data available. And for each region, the model calculated the cost-optimized power grid seven times, using weather data from seven years. This discussion focuses on mean values for cost and total capacity installed and also total values for coal and for natural gas, although the analysis considered three separate technologies for each fuel.

      The results of the analyses confirm that there’s a wide variation in the cost-optimized system from one region to another. Most notable is that some regions require a lot of energy storage while others don’t require any at all. The availability of wind resources turns out to play an important role, while the use of nuclear is limited: the carbon intensity of nuclear (including uranium mining and transportation) is lower than that of either solar or wind, but nuclear is the most expensive technology option, so it’s added only when necessary. Finally, the change in the CO2 emissions cap brings some interesting responses.

      Under the most lenient limit on emissions — 100 grams of CO2 per kWh — there’s no coal in the mix anywhere. It’s the first to go, in general being replaced by the lower-carbon-emitting natural gas. Texas, Central, and North Central — the regions with the most wind — don’t need energy storage, while the other six regions do. The regions with the least wind — California and the Southwest — have the highest energy storage requirements. Unlike the other regions modeled, California begins installing nuclear, even at the most lenient limit.

      As the model plays out, under the moderate cap — 50 grams of CO2 per kWh — most regions bring in nuclear power. California and the Southeast — regions with low wind capacity factors — rely on nuclear the most. In contrast, wind-rich Texas, Central, and North Central don’t incorporate nuclear yet but instead add energy storage — a less-expensive option — to their mix. There’s still a bit of natural gas everywhere, in spite of its CO2 emissions.

      Under the most restrictive cap — 25 grams of CO2 per kWh — nuclear is in the mix everywhere. The highest use of nuclear is again correlated with low wind capacity factor. Central and North Central depend on nuclear the least. All regions continue to rely on a little natural gas to keep prices from skyrocketing due to the necessary but costly nuclear component. With nuclear in the mix, the need for storage declines in most regions.

      Results of the cost analysis are also interesting. Texas, Central, and North Central all have abundant wind resources, and they can delay incorporating the costly nuclear option, so the cost of their optimized system tends to be lower than costs for the other regions. In addition, their total capacity deployment — including all sources — tends to be lower than for the other regions. California and the Southwest both rely heavily on solar, and in both regions, costs and total deployment are relatively high.

      Lessons learned

      One unexpected result is the benefit of combining solar and wind resources. The problem with relying on solar alone is obvious: “Solar energy is available only five or six hours a day, so you need to build a lot of other generating sources and abundant storage capacity,” says Gençer. But an analysis of unit-by-unit operations at an hourly resolution yielded a less-intuitive trend: While solar installations only produce power in the midday hours, wind turbines generate the most power in the nighttime hours. As a result, solar and wind power are complementary. Having both resources available is far more valuable than having either one or the other. And having both impacts the need for storage, says Gençer: “Storage really plays a role either when you’re targeting a very low carbon intensity or where your resources are mostly solar and they’re not complemented by wind.”

      Gençer notes that the target for the U.S. electricity grid is to reach net zero by 2035. But the analysis showed that reaching just 100 grams of CO2 per kWh would require at least 50 percent of system capacity to be wind and solar. “And we’re nowhere near that yet,” he says.

      Indeed, Gençer and Farnsworth’s analysis doesn’t even include a zero emissions case. Why not? As Gençer says, “We cannot reach zero.” Wind and solar are usually considered to be net zero, but that’s not true. Wind, solar, and even storage have embedded carbon emissions due to materials, manufacturing, and so on. “To go to true net zero, you’d need negative emission technologies,” explains Gençer, referring to techniques that remove carbon from the air or ocean. That observation confirms the importance of performing life-cycle assessments.

      Farnsworth voices another concern: Coal quickly disappears in all regions because natural gas is an easy substitute for coal and has lower carbon emissions. “People say they’ve decreased their carbon emissions by a lot, but most have done it by transitioning from coal to natural gas power plants,” says Farnsworth. “But with that pathway for decarbonization, you hit a wall. Once you’ve transitioned from coal to natural gas, you’ve got to do something else. You need a new strategy — a new trajectory to actually reach your decarbonization target, which most likely will involve replacing the newly installed natural gas plants.”

      Gençer makes one final point: The availability of cheap nuclear — whether fission or fusion — would completely change the picture. When the tighter caps require the use of nuclear, the cost of electricity goes up. “The impact is quite significant,” says Gençer. “When we go from 100 grams down to 25 grams of CO2 per kWh, we see a 20 percent to 30 percent increase in the cost of electricity.” If it were available, a less-expensive nuclear option would likely be included in the technology mix under more lenient caps, significantly reducing the cost of decarbonizing power grids in all regions.

      The special case of California

      In another analysis, Gençer and Farnsworth took a closer look at California. In California, about 10 percent of total demand is now met with nuclear power. Yet current power plants are scheduled for retirement very soon, and a 1976 law forbids the construction of new nuclear plants. (The state recently extended the lifetime of one nuclear plant to prevent the grid from becoming unstable.) “California is very motivated to decarbonize their grid,” says Farnsworth. “So how difficult will that be without nuclear power?”

      To find out, the researchers performed a series of analyses to investigate the challenge of decarbonizing in California with nuclear power versus without it. At 200 grams of CO2 per kWh — about a 50 percent reduction — the optimized mix and cost look the same with and without nuclear. Nuclear doesn’t appear due to its high cost. At 100 grams of CO2 per kWh — about a 75 percent reduction — nuclear does appear in the cost-optimized system, reducing the total system capacity while having little impact on the cost.

      But at 50 grams of CO2 per kWh, the ban on nuclear makes a significant difference. “Without nuclear, there’s about a 45 percent increase in total system size, which is really quite substantial,” says Farnsworth. “It’s a vastly different system, and it’s more expensive.” Indeed, the cost of electricity would increase by 7 percent.

      Going one step further, the researchers performed an analysis to determine the most decarbonized system possible in California. Without nuclear, the state could reach 40 grams of CO2 per kWh. “But when you allow for nuclear, you can get all the way down to 16 grams of CO2 per kWh,” says Farnsworth. “We found that California needs nuclear more than any other region due to its poor wind resources.”

      Impacts of a carbon tax

      One more case study examined a policy approach to incentivizing decarbonization. Instead of imposing a ceiling on carbon emissions, this strategy would tax every ton of carbon that’s emitted. Proposed taxes range from zero to $100 per ton.

      To investigate the effectiveness of different levels of carbon tax, Farnsworth and Gençer used the IG model to calculate the minimum-cost system for each region, assuming a certain cost for emitting each ton of carbon. The analyses show that a low carbon tax — just $10 per ton — significantly reduces emissions in all regions by phasing out all coal generation. In the Northwest region, for example, a carbon tax of $10 per ton decreases system emissions by 65 percent while increasing system cost by just 2.8 percent (relative to an untaxed system).

      After coal has been phased out of all regions, every increase in the carbon tax brings a slow but steady linear decrease in emissions and a linear increase in cost. But the rates of those changes vary from region to region. For example, the rate of decrease in emissions for each added tax dollar is far lower in the Central region than in the Northwest, largely due to the Central region’s already low emissions intensity without a carbon tax. Indeed, the Central region without a carbon tax has a lower emissions intensity than the Northwest region with a tax of $100 per ton.

      As Farnsworth summarizes, “A low carbon tax — just $10 per ton — is very effective in quickly incentivizing the replacement of coal with natural gas. After that, it really just incentivizes the replacement of natural gas technologies with more renewables and more energy storage.” She concludes, “If you’re looking to get rid of coal, I would recommend a carbon tax.”

      Future extensions of IG

      The researchers have already added hydroelectric to the generating options in the IG model, and they are now planning further extensions. For example, they will include additional regions for analysis, add other long-term energy storage options, and make changes that allow analyses to take into account the generating infrastructure that already exists. Also, they will use the model to examine the cost and value of interregional transmission to take advantage of the diversity of available renewable resources.

      Farnsworth emphasizes that the analyses reported here are just samples of what’s possible using the IG model. The model is a web-based tool that includes embedded data covering the whole United States, and the output from an analysis includes an easy-to-understand display of the required installations, hourly operation, and overall techno-economic analysis and life-cycle assessment results. “The user is able to go in and explore a vast number of scenarios with no data collection or pre-processing,” she says. “There’s no barrier to begin using the tool. You can just hop on and start exploring your options so you can make an informed decision about the best path forward.”

      This work was supported by the International Energy Agency Gas and Oil Technology Collaboration Program and the MIT Energy Initiative Low-Carbon Energy Centers.

      This article appears in the Winter 2024 issue of Energy Futures, the magazine of the MIT Energy Initiative. More